1. Field of the Invention
This invention relates to a process for producing a foamed polystyrene board. More particularly, the invention relates to a process for producing a foamed polystyrene board having good dimensional stability, high mechanical strength and superior thermal insulation properties.
2. Description of the Prior Art
Foamed polystyrene boards have previously been used widely as thermal insulating material.
In the production of relatively thick foamed polystyrene boards by extrusion, the foam immediately after extrusion from a die orifice is passed through a forming device called a guider to form it into a board of a definite shape. The extrusion formability of the polystyrene greatly affects the operation of the extruder or the quality of the resulting foam. For the production of thick foamed polystyrene board by extrusion, therefore, it is the usual practice to employ polystyrene having much higher flowability than those used generally in the production of thin sheet-like polystyrene foams, for example polystyrene having a melt index, measured by the method described in ASTM D-1238G, of at least about 8.
These polystyrenes having such a high melt index may have improved extrusion-processability, but the resulting foams will have low mechanical strength and inferior dimensional stability.
The foams are generally cut to the desired size, and then shaved to obtain final products. Products having poor dimensional stability would require re-processing or would be useless as insulating material, because variations in dimension occur with the lapse of time. When actual applications are considered, these products should desirably have good dimensional stability not only at room temperatures but at higher temperatures of, say, 70.degree. C. However, conventional foams obtained by using polystyrene having a high melt index do not possess enough dimensional stability to make them useful at higher temperatures.
It is well known to those skilled in the art that the thermal insulating properties of foamed polystyrene are greatly affected by the type and amount of the blowing agent used. For example, Japanese Laid-Open Patent Publication No. 94366/77 discloses a process for producing a foamed polystyrene having superior thermal insulating properties by using a specified type of blowing agent in a specified amount. The foamed products obtained by this process have insufficient dimensional stability, and their thermal insulating properties are not entirely satisfactory. Many additional patents and other documents have been published on the production of foamed polystyrene by an extruding technique. However, the foamed products obtained by the methods disclosed in these documents have their advantages and disadvantages, and none of them exhibit good extrusion formability and a well-balanced combination of the properties required of thermal insulating materials.